Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (High-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) have been used for various applications including but not limited to use as switches in power supplies and power converters, electric cars, air-conditioners, and in consumer electronics, for example stereo systems and communication technology.
In recent years, HEMTs have found wider use in low loss high frequency and low loss high power applications. In particular, Gallium nitride (GaN) based HEMT-devices have been found to be well suited for use in DC rectifiers, power microwave and radar amplifiers, low noise amplifiers, and high temperature elements, etc. Gallium nitride (GaN) material shows a high polarization effect, including spontaneous polarization and piezoelectric polarization. Even without being doped, this polarization effect allows forming a two-dimensional-electron gas (2DEG) adjacent to an interface (heterojunction) of a GaN/AlGaN (gallium nitride/aluminum gallium nitride) heterojunction structure (or GaN/AllnGaN, AlGaN/AllnGaN, aluminum indium gallium nitride). In a 2DEG, the electron concentration is related to the intensity of polarization. 2DEG sheet electron concentration of GaN/AlGaN heterojunction structures can reach very high values. Furthermore, the electron mobility of the 2DEG of GaN/AlGaN-HEMTs is about twice as high compared to the bulk electron mobility of silicon (Si) or silicon carbide (SiC). Therefore, field-effect-transistors based on GaN/AlGaN heterojunction structures are able to control very large currents.
HEMTs are typically normally-on devices using an insulated gate electrode or a Schottky contact for switching, for example a TiN/W-contact on AlGaN. Manufacturing of such structures with Schottky contacts is complicated and not always reproducible. Furthermore, the leakage current of Schottky contacts is often too high for power applications. Alternatively, an insulating gate dielectric such as silicon oxide may be arranged between the undoped AlGaN and a gate electrode used for switching. However, this results in an insulator-insulator interface between the gate dielectric and the undoped AlGaN. This insulator-insulator interface is likely to be charged during operation, in particular at higher temperatures. Thus the threshold voltage of the gate electrode may shift during operation.
Approaches to form normally-off-devices often desired in many applications, i.e. enhancement devices, include a p-doped AlGaN or p-doped GaN barrier layer between the gate and the undoped barrier layer of GaN/AlGaN-HEMTs to raise the conductivity band of the barrier layer such that the threshold voltage of the device is shifted to positive values. In order to obtain functional devices, the undoped barrier layer should not produce a too high density of the 2DEG, which counteracts the threshold voltage rise. However, reducing the density of the 2DEG also reduces the conductivity of the drift region between source and drain.
Accordingly, there is a need to improve heterojunction semiconductor devices.